1. Field of the Invention
The present invention relates generally to increasing the durability of blades in gas turbine engines. In particular, the invention relates to a deformable protective coating applied to shrouded blades to reduce susceptibility to blade airfoil damage caused by the impact between the shroud and airfoil of adjacent blades. The coating acts as a shock absorber by deforming on impact so that the localized impact energy transmitted to the airfoil is reduced.
2. Description of the Known Art
Gas turbine engines having axial flow fans and compressors frequently use mid span shroud projections to provide damping or reduce blade airfoil vibration. The fan or compressor blades have airfoil sections extending radially from a rotor disk. The shroud projections extend circumferentially from each blade airfoil and contact shroud projections on adjacent blades during engine operation. The adjacent shroud projections have opposing mating faces that are in abutting engagement during engine operation. Together, the shrouds on all the blade airfoils engage during engine operation to form an annular stiffening ring. Mid span shrouds have commonly been used on high aspect ratio fan and compressor blades. High aspect ratio blades are relatively long and narrow, having high span length to chord width ratios. Such blades are especially susceptible to aerodynamic flutter, and typically have low resonant frequencies which may be excited at rotor operating speeds. The stiffening ring formed by the mid span shrouds prevents blade aerodynamic flutter, and increases the resonant frequency of the blades.
Examples of blades with mid span shrouds are shown in U.S. Pat. Nos. 3,734,646 issued to Perkins May 22, 1973, and U.S. Pat. No. 4,257,741 issued to Betts et al Mar. 24, 1981. The Betts patent describes a shrouded blade with a pad applied to shroud mating faces. However, the pad in Betts is wear resistant rather than deformable, and does not address reducing damage to blade airfoils due to impact between the shroud and airfoil of adjacent blades.
During engine operation foreign objects may be ingested by the fan and compressor sections. The fan and compressor blades must be designed to withstand such foreign object ingestion with minimum damage to the blade airfoils. During a severe ingestion event, such as a bird ingestion, the blade struck by the foreign object can be damaged. In addition, the sudden loading on the blade can cause the blade shroud to disengage from the shroud on the adjacent blade and slide forward to impact against the adjacent blade airfoil. The impact of the shroud against the adjacent blade airfoil can result in severe localized impact loads and airfoil damage requiring the adjacent blade to be replaced. In extreme cases, blade failure can occur, requiring engine shutdown due to vibration caused by out of balance loads.
One possible approach to reducing airfoil damage during foreign object ingestion is to thicken the airfoil section. However, thickening the airfoil section is undesirable because it adds weight to the engine and can affect the aerodynamic performance of the blade airfoil. As a result, engineers and scientists continue to seek better methods for increasing the foreign object damage tolerance of blades used in gas turbine engines.